The present invention relates to an electronic control for the commutation of brushless DC motors and further provides means to vary the speed of the motor by varying the duty cycle of electrical conduction within a phase winding of the motor. In addition to duty cycle variations, the timing of the conductance is such that it will occur in an optimum range within the torque cycle of a phase winding to maximize motor efficiency over a wide speed range.
The herein-described motor control circuit utilizes a centered pulse turn-on concept which is adaptable to operation with any number of phases in a motor. Each phase has its own on-time and delay time data generation. Based on this timing data each phase winding is energized only for a carefully selected portion of the time said phase winding is capable of generating torque for the desired direction of rotation of the rotor. Multiple phases, even if the torque cycles are overlapping, may be energized independently when most appropriate for each. The herein-described control could also be connected to energize each phase winding for the identical period during motor rotation such that all phases are energized for the same duty cycle adjusted to be initiated at the correct start time.
It has been recognized that torque in a unipolar driven motor may be applied by directing current in a single direction through a winding of the motor at a selected time relative to the rotor position. The force applied to the rotor with respect to the current applied, increases and decreases during this interval in which torque will be produced. By sensing the rotor position through the rotational angle during which current may be applied, it is possible to apply appropriate electrical power at the point where it is most efficient. By adjusting the length of time the electrical power is applied and the delay interval from the beginning of the potential torque producing angle to the time when the current is actually applied, it is possible to control both the amount of power applied and the speed of the motor.
An advantage of using a digitally-controlled motor speed control is that all inputs and outputs are compatible for use with a microprocessor control.
Although the invention is described relative to a unipolar-driven motor, it is likewise applicable for a bipolar-driven motor. Some additional logic may be necessary to determine when to appropriately apply current in a reverse direction to achieve the bipolar effect. Additionally, it may be desirable to have additional position sensing devices to indicate a different point on the motor's torque cycle on which the delay interval should begin.